SEPARATION OF THE INTRINSIC PRESSURE EFFECT ON T(C) OF YBA2CU3O6.7 FROM A T(C) ENHANCEMENT CAUSED BY PRESSURE-INDUCED OXYGEN ORDERING

The superconducting transition temperature T(c) of polycrystalline YBa2Cu3O6.7 has been investigated under purely hydrostatic He-gas pressure. In a series of experiments, it has been shown that the degree of oxygen ordering depends on the applied pressure and the storage temperature T(s) of the sample. At temperatures below almost-equal-to 240 K, there is no indication for oxygen ordering. When pressure is applied at such low temperatures, the T(c) values show no relaxation effects. and (dT(c)/dp)i = 7.4 K/GPa is observed which we believe is intrinsic. At temperatures above 240 K, the oxygen sublattice reacts reversibly to the applied temperature and pressure. Changes of the storage temperature or pressure cause T, relaxations with time constants that are temperature dependent. When the relaxations were completed the effects of pressure and storage temperature on T(c) are independently deter-mined to be : (1) a pressure-independent effect of the storage temperature T(s) with dT(c)/dT(s) = -0.02, (2) a temperature-independent effect of pressure-induced oxygen ordering with (dT(c)/dp)O = 4.1 K/GPa. For temperatures above 240 K, the intrinsic and the oxygen ordering effect add to dT(c)/dp = 11.5 K/GPa.